c++マルチステート詳細例
5826 ワード
*C++マルチステートはコンパイル時マルチステートと実行時マルチステートに分けられます.*コンパイル時のマルチステートは、例1.*実行時のマルチステートは、例2.*キーワードvirtualはfunction declarationまたはclass declarationに使用できます.*キーワードvirtualが関数を修飾するときに虚関数を宣言します.*キーワードvirtualがクラスを修飾するときに虚ベースクラス(virtual base class)を宣言する.虚基類の役割は空間を節約し、重複と二義性を避け、多態とは関係ない.*抽象クラス(abstract class)=純粋な虚関数(pure virtual function)を含むクラス(自分で宣言しても継承しても)、例6.1.一般メンバー関数への呼び出しはポインタ/参照のタイプによって決まる(コンパイル時マルチステート)
#include <iostream>
using namespace std;
class B
{
public: void vf() { cout << "This is class B" << endl; }
};
class D: public B
{
public: void vf() { cout << "This is class D" << endl; }
};
main()
{
B b, *pb;
D d, *pd;
pb = &b; pb->vf();
pb = &d; pb->vf(); // no cast needed
pd = (D*)&b; pd->vf(); // must cast explicitly
pd = &d; pd->vf();
}This is class B
This is class B
This is class D
This is class D#include <iostream>
using namespace std;
class B
{
public: void virtual vf() { cout << "This is class B" << endl; }
};
class D: public B
{
public: void vf() { cout << "This is class D" << endl; }
};
main()
{
B b, *pb;
D d, *pd;
pb = &b; pb->vf();
pb = &d; pb->vf();
pd = (D*)&b; pd->vf();
pd = &d; pd->vf();
}This is class B
This is class D
This is class B
This is class D#include <iostream>
using namespace std;
class B
{
// non-virtual function may be undefined as long as it's not called
public: void vf();
};
class D: public B
{
public: void vf() { cout << "This is class D" << endl; }
};
main()
{
B b, *pb;
D d, *pd;
pd = (D*)&b; pd->vf();
pd = &d; pd->vf();
}This is class D
This is class D#include <iostream>
using namespace std;
class B
{
// virtual function in base class must be defined or be pure
// unless the base class and all its derived classes are not instantiated
public: void virtual vf();
};
class D: public B
{
public: void vf() { cout << "This is class D" << endl; }
};
main()
{
D d, *pd;
}error LNK2001:
unresolved external symbol "public: virtual void __thiscall B::vf(void)" (?vf@B@@UAEXXZ)#include <iostream>
using namespace std;
class B
{
public: void virtual vf() { cout << "This is class B" << endl; }
};
class D: public B
{
// virtual function in derived class may be undefined as long as
// the derived class is not instantiated
public: void vf();
};
main()
{
B b, *pb;
D *pd; // instantiation like "D d;" or "D *pd = new D;" causes link error
pb = &b; pb->vf();
pd = (D*)&b; pd->vf();
}This is class B
This is class B#include <iostream>
using namespace std;
// B is abstract class because its vf() is pure virtual function
class B
{
public: void virtual vf()=0;
};
// C is also abstract class because it inherits pure virtual function vf()
class C: public B
{
// "public: void vf();" here would change vf() to be non-pure virtual
// function and cause link error when D instantiates
// but if D derives from B directly as "class D: public B" then it is OK
};
// D is not abstract class because its vf() is defined and no longer pure virtual function
class D: public C
{
public: void vf() { cout << "This is class D" << endl; }
};
// E is not abstract class and has another implementation of vf()
class E: public D
{
public: void vf() { cout << "This is class E" << endl; }
};
main()
{
B *pb; // instantiation like "B b;" or "B *pb = new B;" causes link error
C *pc; // instantiation like "C c;" or "C *pc = new C;" causes link error
D d, *pd;
E e, *pe;
pb = &d; pb->vf();
pc = &e; pc->vf();
pd = &e; pd->vf();
pe = (E*)&d; pe->vf();
}This is class D
This is class E
This is class E
This is class D